Superconductivity, the virtual disappearance of electrical resistivity, was initially discovered in mercury cooled to the boiling temperature of liquid helium. This discovery initiated the search for materials which would be superconductive at higher temperatures. In 1987 came a significant advance. Superconductivity was found at 95.degree. K. in a material composed of several phases containing yttrium, barium, copper and oxygen. The discovery was significant in that the temperature at which superconductivity appeared was above the boiling temperature of liquid nitrogen, which could then be used for the cooling medium. The superconducting phase was found to correspond to the crystalline, orthorhombic oxide YBa.sub.2 Cu.sub.3 O.sub.7. The superconductive property was lost, however, upon heating the orthorhombic phase under conditions where oxygen was depleted giving rise to a tetragonal phase, the composition of which was close to YBa.sub.2 Cu.sub.3 O.sub.6. The transition seemed to occur around the composition YBa.sub.2 Cu.sub.3 O.sub.6.5. Hence the superconductive property exists in compounds of the formula YBa.sub.2 Cu.sub.3 O.sub.7-x where x may vary from 0 to 0.4, the optimum being about 0.19.
Other high temperature superconductors which now have been identified include YBa.sub.2 CuO.sub.7-x, Ba.sub.x La.sub.5-x Cu.sub.5 O.sub.5(3-y), Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+x, Bi.sub.4 Sr.sub.3 Ca.sub.3 Cu.sub.4 O.sub.x, and Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.x.
Superconductive metal oxide material can be produced by traditional ceramic techniques of grinding metal compounds in stoichiometric ratio to bring the metal compounds into proximity. Subsequent calcination allows the metal ions in their respective crystalline compounds to diffuse into the others. Repeated regrinding and calcination under controlled conditions produces the desired phase which has the superconductive property.
Most of the prospective applications of superconductors are based on the capability of transmitting electric power loss free, and on the production of powerful, compact magnets. Because motors and generators are based on magnetism, there is great potential for reducing their weights, sizes and inefficiencies. Powerful magnets are conceived to allow the suspension of objects such as a shaft in a bearing and a train over a track.
The superconductive metal oxides, like ceramics, are intrinsically brittle and their fabrication into useful shapes, even basic wire, presents many challenges. The most practiced method to date for the formation of superconductive wires has been the powder-in-tube technique. The superconductive material in powder form is packed into a silver, copper or stainless steel tube. The tube is then swaged and drawn, or rolled, down to a small diameter which can be further formed into a useful configuration.
Lusk et al. in Supercond. Sci. Technol. 1, 137 (1988) reported on the fabrication of a ceramic superconducting wire by an extrusion method. Superconductor precursor material in powder form was mixed with a binder such as epoxy resin, and the mixture was extruded into a wire form. The extrusion was heated in a nonreactive atmosphere to remove the binder, and then sintered at high temperature in air or oxygen to develop strength and the superconductive phase. Fragile wire with a diameter of about 0.8 mm resulted from this method.
The preparation of superconductive fibers by extruding or spinning a polymer-metal precursor was described by Chien et al. in Physical Review B, 38, 1953 (1988). Metal ions in the desired atomic ratios were complexed to a polymer. The polymer solution was extruded, dried and wound on a mandrel. Heating in nitrogen pyrolyzed the polymer, and subsequent heating in oxygen converted the metal intermediates to the superconductive oxide. The process produced fibers having diameters of 1 to 100 microns and grain sizes from 1 to 50 microns.
Jin et al. in Appl. Phys. Lett. 51, 943 (1987) described three different laboratory fabrications of YBa.sub.2 Cu.sub.3 O.sub.7-x wire by molten oxide processing. In the melt drawing technique, the center of a bar of YBa.sub.2 Cu.sub.3 O.sub.7-x material was fused with a laboratory blow torch flame, and the two unmelted ends pulled apart leaving a 1.2-mm diameter filament between them. In the melt spinning technique, one end of a bar of YBa.sub.2 Cu.sub.3 O.sub.7-x material was heated and a molten droplet allowed to fall on the outside of a rotating mandrel producing a ribbon 1.5 mm wide and 0.3 mm thick. Still another experiment employed a silver wire as a substrate onto which YBa.sub.2 Cu.sub.3 O.sub.7-x powder in a binder was deposited. The composite was dried, producing a 0.75-mm diameter composite wire containing an 0.25-mm diameter metal core. The wire was further processed by rapidly moving it through a torch flame and melting the outer portion. The wire formed in each of these three methods required a homogenizing heat treatment followed by an oxygen heat treatment to develop the superconductive phase. In production, any of these three techniques would require a high temperature melting furnace and precise control of operating variables.
The processes described above were all directed to the fabrication of a single filament. A process for producing metal oxide fibers, textiles and shapes was described by Hamling in U.S. Pat. No. 3,385,915. By textiles is meant a variety of textile forms including single filament, staple fibers, continuous tow and yarns, woven fabrics, batting and felts composed of fibers.
The Hamling process comprises initially impregnating a preform of organic polymeric textile material with one or more compounds of metals as desired in the final product. The impregnated material is heated under controlled conditions which prevent ignition of the organic material, but pyrolize the organic material to predominantly carbon and remove the carbon as a carbon-containing gas. The heating continues to oxidize the metal compounds. At least part of the heating is performed in the presence of an oxidizing gas. A product results which has substantially the same shape as the preform, but only about 40% to 60% of its original size. The metal oxide in the product typically is substantially micro-crystalline, or amorphous, that is, its crystallites are so small that they are barely discernible by conventional x-ray diffraction. This is indicative of a crystallite size on the order of 0.1 microns or less, which Hamling preferred for maximum strength in his product. The process, however, is described as capable of preparing fibers with crystallite sizes up to approximately 1 micron. With larger crystallite sizes, a significant loss in strength occurred. Mechanical properties of the product were impaired when the crystallite size exceeded approximately one-tenth the diameter of the fibers.
It is known that material capable of superconductive behavior must be in a crystalline state. Hence the process as described by U.S. Pat. No. 3,385,915 would not produce superconductive metal oxide.
Fabrics composed of metal oxides are described by Hamling in U.S. Pat. No. 3,663,182. Such fabrics are produced by the process described in U.S. Pat. No. 3,385,915, which has been summarized above. Hence, the fabric has all the characteristics of a product of that process, and would not be expected to have superconductive properties.
It is an object of the present invention to provide a process for producing superconducting metal oxide fibers, textiles and shapes. It is also an object to produce these products with flexibility and strength so as to allow their further shaping.
It is a feature of this invention that the starting material is organic polymeric material which can be preformed into the final product shape.
It is an advantage of this process that complicated and irregular product shapes can be produced from inexpensive organic materials which are readily preformed into the desired final shape. The preforming is inexpensive in that costly machining is unnecessary. Another advantage is that a high temperature melting furnace is not required.